The problem probably relates to the source of another question: Itgennnl005 bij aanroepen Nmbrs in Invantive Cloud (Dutch).
Over the weekend a switch was made to a new cluster running Invantive Cloud (the website, Data Guard, App Online and Bridge Online still run on the previous infrastructure).
The registration of some metadata elements triggers an error currently. This will be resolved during the next hours.
Please try again in four hours on Invantive Cloud.
There is no impact on downloads of Loket over Invantive App Online and Invantive Bridge Online.